The invention generally relates to electromagnetically detecting thin resistive bodies in shallow water and terrestrial environments.
One technique to locate an oil reservoir beneath the sea floor is to measure electromagnetic fields that are produced by a controlled electromagnetic source. More specifically, in a technique called controlled source electromagnetic (CSEM) surveying, an electrical dipole (i.e., a controlled electromagnetic source) may be towed by a surface vessel a short distance above the sea floor. Measurements of the resulting electric and/or magnetic fields are then measured using receivers, which may be deployed, for example, on the sea floor. Ideally, the presence of a thin resistive layer, such as an oil reservoir, affects the measured electric and magnetic fields in a way that can be detected from the measured data.
CSEM surveying typically is limited to deep water, as a phenomenon called an “air wave effect” (as referred to in the literature) currently limits the use of CSEM surveying in shallow water environments. More specifically, the electromagnetic fields that are produced by the electric dipole interact with the air-sea interface to generate electromagnetic energy, or “air waves,” which diffuse from the sea surface down to the receiver. For shallow water, the air waves dominate the measured electromagnetic data so that the presence of a thin resistive body may not be readily discernible from the data. Similar challenges limit the application of CSEM surveying to terrestrial environments.
Thus, there is a continuing need for better ways to process data that is generated by controlled source electromagnetic surveying in shallow water and terrestrial environments.